The present invention relates generally to polymer foam planks and, more particularly, to a foam plank with a densified skin and a method and an apparatus for making the densified skin foam plank.
Foam structures are useful as protective packaging. Protective packaging systems have employed foam structures to suspend a product in the center of a container during shipping and storage. For example, foam end caps fit on opposite ends of a product such as a computer, printer, computer monitor, medical monitoring device or other fragile electronic equipment, to protect the product from shock and vibration damage. In general, protective packaging foams are low density polymeric materials with good physical properties capable of supporting the product weight without excess deformations during package transit and storage. The exact foam density required for a particular application depends on the foam compression properties, shear properties, shock mitigation properties during package drops, creep properties, buckling limits, and thickness resiliency with multiple drops.
Laminated foam structures have been developed that provide enhanced foam physical properties and/or simplify the creation of fabricated end caps for protective packaging systems. Some laminated foam structures comprise a low density foam core with one or more layers, and one or more skins of high density foam laminated to the core. U.S. Pat. Nos. 5,876,813 and 5,882,776 describe examples of such laminated foam structures. In addition, other laminated foam structures may comprise a low density foam core with one or more layers, and one or more skins of thin polymeric film or polymeric sheet. These laminated foam structures provide the desired property enhancements.
Conventional lamination techniques produce the laminated foam structures, including the techniques of bonding the layers using heat, film, or applied adhesives. One problem with the laminated foam structures is their cost and inefficiency of production. The lamination process for the laminated foam structures first requires the production and stocking of various rolled foam sheet materials or individual foam planks having different properties, densities, dimensions, and colors. These foam materials, after a curing time, are then laminated on separate lamination equipment to form the finished product. Laminated foam structures with polymeric films and/or sheets also require a separate production process. Often, polymeric films and sheets are produced off-site from the foam and laminate production location. Therefore, film rolls and sheets must be transported to the lamination plant. Another problem with the conventional lamination techniques is that an inventory of component foams, films and/or sheets must be maintained to meet manufacturing scheduling. Furthermore, all component materials must be inspected and certified to meet quality and product specifications on an individual basis before being used at the lamination production stage.
Thus, there is a need to develop foam structures having enhanced properties. There is also a need to manufacture these foam structures with a relatively simple production process that does not have the inefficiencies and complexities associated with the conventional laminated product production processes. The present invention is directed at satisfying these needs.
In accordance with one aspect of the present invention, there is provided a method for making a densified skin polymer foam plank. The method comprises providing a polymer foam having a plurality of cells and heating an outer surface of the foam. The heating step collapses and melts the cells adjacent the outer surface to form the densified skin having a density greater than a density of the foam before the heating step. Additionally, the method further includes a step of extruding the polymer foam from an extruder. Prior to extruding the foam, the method may further include the steps of mixing and melting a polymer mixture comprising at least 50 weight percent low density polyethylene, mixing the mixture with one or more blowing agents, cooling and pressurizing the mixture.
In accordance with another aspect of the present invention, there is provided an apparatus for making a densified skin foam plank. The apparatus comprises a heating station capable of heating an outer surface of an ethylene polymer foam plank to form a densified skin. The densified skin has a density at least ten times the density of the ethylene polymer foam before the heating. The heating station may comprise hot air knifes. The apparatus may further include an extruder capable of producing the ethylene polymer foam out of an extrusion die. The extruder may be in-line with the heating station to continuously or intermittently provide the ethylene polymer foam to the heating station.
In accordance with a further aspect of the present invention, there is provided a densified skin ethylene polymer foam plank. The plank comprises a low density ethylene polymer foam having dimensionally stable closed cells. The plank also includes a densified skin on a top surface of the foam. The densified skin comprises melted and collapsed foam cells formed by heating the top surface of the foam.